In Morrowind when one asks around about joining the legion you are told about the Gnisis garrison is accepting recruits and they also refer to it as a deathshead legion fort. One takes that the Imperial Legion in game was inspired so loosely based on the real life ones so:
A) The Imperial Legion is the title for the whole organisation
B) There are multiple legions and the Deathshead Legion is the paticular one that has been posted to Vvardenfell

Would this be correct?

I think it's correct now.

I dont think i have seen mention of any other names ingame, what would be the evidence that there is more than one legion on the island?

Because the characters who are currently in the Morrowind Legion directly refer to the Deathshead Legion at Gnisis.

As an example to illuminate, you walk into a Mage's Guild with the intention to join the guild. No one is aparrently around so you wander around a little. You walk into large wing and encounter a person. For the sake of this example, you know he is a member of the guild. You ask if you can join the Mages Guild. The person then responds by saying that 'you might want to try the Mage's Guild upstairs.' This statement forces you to conclude that either the wing you are currently in is not a part of this guild hall or that there is a different Guild branch upstairs that handles recruitment. It forces you to conclude that because the person felt that he had to distinguish his guild membership from the membership of those upstairs.

When you apply this logic to the information you're looking for it would seem silly if a Legionnaire at Ebonheart was a member of the Deathshead Legion at Ebonheart and she told you to join the Deathshead Legion at Gnisis. She would instead tell you to join the Legion at Gnisis.

Additionally, even though I have not completely played through the Legion quests in Morrowind, I have not encountered any other Legion names.

Apparently the Legion which tried to invade Akavir was called the 10th Legion, so maybe most of them are numbered and names like "Deathshead Legion" are a nickname.

Wasn't there something in Redguard about the Colovian legion being called the Red Legion?

I think it's confusing because the army as a whole can be referred to as the Imperial Legion, when it is really the Imperial Legions. But yes, the Death's Head legion is only one group of soldiers in the Imperial Army.

But it's strange that they seem relegated to Gnisis, and you never hear the names of the other legions, is it one legion per fort, etc? I can't recall how large a legion is (10,000 men?) but I know that historically a Caesar had one legion in Spain, and 3 in Britain, even though it was smaller and had less people, they were just more trouble.

So it would seem that Morrowind, as a troubled province, would have more than one, but very unlikely that there is one for every single fort.

Unknown to Berylith but known to him now, his servant had once ridden with the legendary Cursed Legion of High Rock. He not only knew how to fight in Orcish armor himself, but he had acted as trainer to other Orcs before retiring into domestic service. Desperate, Bowyn immediately engaged him as his full-time trainer.

This legion wasn't stationed in Morrowind, but it had a nickname.

But each fort might be the HQ for one. After all the Legionaries at Seyda Neen, Caldera and Wolverine Hall presumably belong to one. Perhaps in times of peace the Legions are kept understrength as a cadre. Would explain why there are lots of agents, champions etc and not many spearmen and troopers.

Don't you? Ever seen the roadsigns to "Moonmoth Legion Fort", "Buckmoth Legion Fort" etc.?

And it continues the moth name theme of Deathshead Legion. I presume it is one per fort, and just due to a semantic/gameplay issue, you don't see the amount of men that would amount to what we call 'a legion' in real life in game.

Thanks for the intresting replies.

I had forgotten about the forts, i had always looked at them as just names i.e. Fort Ticonderoga; so in these cases a name followed by what they actually are i.e "name" Legion Fort. Although i guess you could interpret it the opposite way like you have suggested impling there is at least 4-5 legions on the island?

Well, it is during a time of increased ash storms, blight, dumner hostility to each other, and other Dagoth Ur related things. But, you are right, 4-5 legions does sound a bit excessive.

I don't think so, but I'd need to look it up.

If you talk to some of the general population and ask about 'latest rumours', once in a while you will encounter an answer going something like: "The trouble back in the mainland might require the Empire to retreat the legionS from Vvardenfell" or some such.

edit: of course a Rumour is nothing to really base facts upon, but it does at least show that the general population believes that there are several, which is quite likely then.

They said that all the garrisons were full and that Gnisis was the only one recruiting. Could they have been multiple legions? Maybe, but the game doesn't distinguish that very well. Also, why would one legion be called in to do the other legions duties? It doesn't make much sense to me.

I think that perhaps Deathshead is simply a nickname given to the Legion garrison at Gnisis. This also should imply that there is only one legion currently occupying Vvardenfell.

That appears to be either a nickname for the Legions as a whole or Tiber Septim's elite Legion which personally guarded him, most of them Nordsmen.

MK wrote a brief retcon mentioning them to explain the in-game Cyrodill climate:

http://www.imperial-library.info/obscure_text/many-headed_talos.shtml

"And after the throne of Alinor did finally break at the feet of Men, and news of it came to the Dragon Emperor in Cyrodiil, he gathered his captains and spoke to them, saying:

"'You have suffered for me to win this throne, and I see how you hate jungle. Let me show you the power of Talos Stormcrown, born of the North, where my breath is long winter. I breathe now, in royalty, and reshape this land which is mine. I do this for you, Red Legions, for I love you.'"